I am about to install a alarm system in my 2007 cobalt. My car has the single wire door lock and unlock setup. So lock is straight ground and unlock is ground with a 1.5k resistor. Also with the trunk it is a negative trigger. The alarm module does no have on board relays for the door or the trunk and gives you instructions how to wire the external relays. The alarm is outputting a negative on the door lock and the aux output already that is why I am questioning why I need a relay. I have my resistor ready for the door unlock. The wiring for the relay basically has 12 going in all the time the negative triggering the relay from the alarm module box then that brings the new ground into play connecting it to the bcm. But if the ground from the alarm module is enough to trip the relay why is the ground not enough to just trigger the door locks?
Thanks for the help
The negative output for the door lock/unlock from the alarm is a transistorized output. It does not always give a good ground signal. If it doesn't then there is resistance to ground. When you connect an inline resistor to this, the 1.5K value could be way off and the BCM won't accept the signal as a valid unlock trigger. Using a relay will give you a true ground without resistance so the resistance value will be accurate. You may be able to get away without using a relay but it might not work once in a while. Relays are cheap and easy to wire in so if you never want to have a problem, use one. Some manufacturers vehicles are more tolerant than others to resistor values. Many Chrysler vehicles you must use a relay as they have tight tolerances to resistance values. Some alarms also use dual transistors. The same wire for negative unlock gives a 12 volt positive pulse to lock. If you connect both the lock and unlock (with resistor) wires to the one vehicle lock/unlock wire, the BCM won't activate the locks. Older Audiovox units were like this. The relay isolates the two outputs from the BCM.
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sparky
Thanks for the help. Makes perfect sense to me. I wired 3 up last night in my living room using some regular under the hood relays I had laying around hopefully they work.
Why do I need a relay? My question exactly. I am installing a Viper 5706V in a 2001 3rd gen Toyota 4Runner.
I have been all over the WEB and now understand the “basics” of Relays. I understand perfectly why they are needed in something like a Headlight circuit. And, understand why they would be needed to add electric door locks to a vehicle without them. I also realize the Viper itself doesn’t put out enough power to do much directly.
But, if I understand what I have been finding, I need a relay to make my horn honk for instance, even though it already has a relay that is activated by a switch. Why can’t I just “activate” the switch with the alarm output (understanding that I may need some diodes to – isolate or protect the circuit/switch).
Thanking you in advance.
Bob, your Toyota should have a horn wire in the steering column. This is a low current NEG and the 5706 BROWN / black will drive it directly, no relay required.
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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.